Microcellular shoe soles have become common, particularly for athletic shoes, but increasingly for other shoes as well. In contrast to solid elastomers which were traditionally used before the advent of microcellular elastomers, microcellular shoe sole components offer the opportunity to manufacture shoes with increased cushioning and lighter weight. Although early microcellular components often attained these advantages at the expense of durability and wear resistance, great strides have been made in these areas, and now the use of microcellular components is virtually universal. Microcellular polyurethanes are particularly well suited for these uses.
In the molding of microcellular shoe sole components, prepolymer systems are generally used. Such systems include an isocyanate-terminated prepolymer, prepared by the reaction of 4,4'-methylenediphenylene-diisocyanate (4,4'-MDI, "pure" MDI) or MDI variants with a polyoxypropylene diol, and cured by reaction of the prepolymer with a diol chain extender, e.g. 1,4-butanediol. A minor amount of water, in conjunction with a catalyst which catalyzes the CO.sub.2 -producing water/isocyanate reaction, provides the necessary blowing to produce the small, uniform cells characteristic of microcellular elastomers. The density is generally between about 0.2 g/cm.sup.3 and 0.5 g/cm.sup.3, considerably less than non-cellular polyurethane elastomers, but considerably higher than cellular polyurethanes such as flexible and high resilience flexible foams.
Despite the great improvements which have been made in microcellular polyurethane shoe sole components over the past years, room for considerable improvement still exists. For example, production speed is limited by the time necessary to achieve the requisite "green strength" which allows the molded component to be removed from the mold without tearing and without suffering permanent deformation. Lowering demold time through increased catalyzation is possible, however, the processing window generally suffers as a result. Increased catalyzation can also negatively affect physical properties. Use of high primary hydroxyl polyols, commonly used to increase reactivity of conventional polyurethane foams, is of no help in prepolymer-based microcellular foams, as the polyol component has been prereacted into the prepolymer, and thus the higher reactivity of the primary hydroxy group is irrelevant.
Physical properties of the molded foam product are also important. Improvements in comfort-related properties such as resiliency, support factor, and the like, as well as lowered density, are well known areas where continual efforts toward improvement are being made. Less well known, however, are such factors as tensile strength, elongation, and tear strength. While each of these is to some degree related to desirable end-use physical characteristics, these properties are important in processing as well. For example, a microcellular formulation which produces a product having higher ultimate tensile and tear strength will reach the level of physical properties (green strength) necessary to allow demolding more quickly as compared to a formulation producing microcellular foams having lower ultimate physical properties. The higher tensile and tear strengths will also allow removal from the mold with less frequency of damage, reducing the scrap rate during the molding process. Such characteristics are also required of the demolded, fully cured products, which frequently must be stretched, pulled, or pushed into various shoe cavities, coverings, etc.